This invention relates generally to rotary expansible chamber devices having variable displacements (i.e. variable displacement pumps and motors) and more specifically to a control system for such devices which possesses new and unique operating capabilities.
Hydraulic motors are employed in many applications where substantial amounts of power must be delivered at relatively low shaft velocities. Examples of typical uses includes winch drives, agricultural and earth moving machinery drives, propeller drives, and rotary drives for large machines.
Speaking in a general way, it is possible to control a hydraulic motor's output characteristics (i.e. shaft speed and torque) by controlling the flow rate and pressure, respectively, of the hydraulic fluid which powers the motor. Unsophisticated forms of this type of control can be wasteful of hardware and power, and they may not even meet the desired control objectives. For example, that sort of control may comprise the inclusion of separate pressure and flow control devices in the hydraulic fluid power line to the motor with the pressure and flow control functions being accomplished by these devices. Moreover, operation of these control devices typically is by means of electro-hydro-mechanical devices which are calculated to perform the control functions, but the level of system performance which is obtainable may fall short of the desired control objectives or may not be cost effective. More sophisticated forms may require expensive servo valves and controls and/or special types of pumps and controls.
Certain types of hydraulic motors comprise means for mechanically adjusting the motor's characteristics. Two important types are the radial piston type and the axial piston type. In each of these the ratio of displacement volume to output shaft rotation may be mechanically adjusted over a given range so as to provide within said range any desired ratio. In other words, they are variable displacement motors. Control systems may be associated with these motors to provide for this adjustment on a continuous basis so that a continuous control capability is attained. Stated another way, the mechanical adjustment mechanism in such a motor is capable of adjusting the mechanical advantage of the cylinder's pistons on the output shaft. As the mechanical advantage increases, the ratio of displacement volume to shaft rotation also increases, and at constant pressure and flow, the increasing mechanical advantage creates an increasing output shaft torque and a decreasing output shaft speed. Similarly as the mechanical advantage decreases, the displacement volume to shaft rotation ratio also decreases and at constant pressure and flow, the decreasing mechanical advantage creates decreasing output shaft torque and an increasing output shaft speed. In the ensuing description the term "mechanical advantage" will be used in this sense.
In an axial type hydraulic motor, the mechanical advantage is adjusted by means of a swash plate coupling of the cylinder pistons to the motor output shaft. The swash plate angle is adjustable to adjust the relationship between piston travel within the cylinders and the motor shaft axis to thereby adjust the mechanical advantage. In a radial cylinder type motor one construction for adjusting the mechanical advantage is by means of an adjustable eccentric coupling between the cylinder pistons and the output shaft. The degree of eccentricity of an eccentric about the motor output shaft is adjusted over an adjustment range to provide a corresponding change in the mechanical advantage. In both radial and axial cylinder motors, the output shaft speed is essentially directly proportional to the volumetric flow of pressurized hydraulic fluid power into the motor for a given mechanical advantage.
An especially good example of a radial piston motor, and the one which is employed in the preferred embodiment of the present invention, is disclosed in U.S. Pat. No. 3,828,400 issued Aug. 13, 1974. The motor of that patent is marketed under the trade name Staffa, and it has enjoyed significant commercial success even to the point where it has been substantially copied by other manufacturers. In the motor of the type shown in U.S. Pat. No. 3,828,400 an eccentric in the form of a ring is disposed around the output shaft. Pistons within the radial cylinders are coupled via connecting rods to the eccentric with the radially inner ends of the connecting rods having slippers which are disposed against the radially outer surface of the eccentric. The eccentric has keyed connections with the output shaft so as to be rotatably coupled with the output shaft but to be bodily displacable radially of the output shaft to vary the eccentricity. The radial adjustment of the eccentric is performed by means of hydraulic actuated control pistons disposed within radial bores of the output shaft which act against the inner periphery of the eccentric.
In a control system for this type of motor an electrically controlled hydraulic valve is connected with the control pistons so as to control the application of hydraulic control fluid to the control pistons and thereby adjust the position of the eccentric.
As a practical matter, most motor applications do not involve a constant steady state mode of operation. Accordingly, adjustment controls for a motor are a virtual necessity. For a hydraulic motor, two independent control parameters which are commonly used are flow and pressure. It will be appreciated of course that these two particular parameters are related with other parameters including speed, torque, and horsepower and that known mathematical formulae define the relationships. For example, the output shaft horsepower is equal to the product of the output shaft speed and the output shaft torque. The horsepower output of the motor is equal to the horsepower input to the motor times the efficiency. In hydraulic motor systems, this efficiency can be quite high. Since the product of pressure times flow is equal to the input horsepower to a hydraulic motor, the output shaft speed and pressure of hydraulic power fluid to the motor are two independent parameters which may be monitored and adjusted to control the horsepower output of the motor.
In many control applications, it is desirable to either provide an infinitely variable speed control which is operable to maintain a set output shaft speed in spite of load changes within a given range, or a constant horsepower control which is operable to maintain a constant horsepower output for a variable load over a given range.
The present invention comprises a hydraulic motor control system which embodies both of these capabilities in a new and unique way. It possesses an operating characteristic which can maintain a desired set speed for the motor output shaft within a given load range but which in response to load increases beyond said given range assumes a constant horsepower mode of operation. Adjustable controls are provided to set desired control parameters, speed and pressure in the disclosed embodiment, and other controls are additionally provided to establish maximum limits for these parameters.
In general, for a given speed setting a certain range of loads can be driven by the motor without any need to adjust the motor's displacement because the pressure can rise and fall to handle increases and decreases in load within this certain range. If on the one hand there is a fluctuation which gives rise to a certain incipient increase in shaft speed, that incipient speed increase is effective, via the control, to regulate the shaft speed back to the speed setting by causing the motor's displacement to increase in an appropriate amount. Even though the increased displacement is inherently accompanied by an increase in the mechanical advantage of the motor, that increase is not incompatible with the load on the motor because the pressure will stabilize at an appropriate level. In other words, even though the increase to mechanical advantage by itself would tend to increase the torque output of the motor, the final pressure will be appropriate to the motor load at the increased displacement for maintaining the desired speed.
On the other hand, should the motor load increase from a load within the range which can be driven by the motor at the set speed so as to exceed the maximum of that range for that set speed, the control operates in a constant horsepower fashion. Exactly how the control arrives at constant horsepower operation depends upon a number of factors including the specific control electronics (including their calibration and adjustment) and the exact nature of the load increase. The load increase is reflected as incipient pressure increase and/or speed decrease. With the specific electronic control circuit to be described later, the control, at least initially in response to load increase, seeks to maintain a speed control mode of operation by the electronics commanding a decrease in the displacement before the mode enters constant horsepower.
For certain types of load changes, decrease in the motor displacement indeed occurs thereby attempting to increase the shaft speed so as to correct for the incipient speed reduction. Such an initial adjustment for purposes of speed correction reduces the mechanical advantage in the wrong direction for achieving increased torque which is required for the increased load. Since an increased load will typically occasion an increase in the pressure even if no reduction in the motor's mechanical advantage is made, any initial reduction in the motor's mechanical advantage for speed regulation purposes is therefore effective to augment the pressure increase so that the pressure rise is even more rapid than would be the case if no reduction in mechanical advantage took place. When a set pressure level has been equalled or exceeded, the control transfers from the previous "speed control" mode providing constant speed output to a new "horsepower control" mode providing constant horsepower output.
For more extreme load changes the response of the mechanical advantage adjustment is not fast enough to cause any actual reduction in displacement before the constant horsepower mode takes over. Such more extreme load change will have resulted in the pressure rising such that the constant horsepower control mode essentially instantaneously takes effect so that the mechanical advantage begins immediately to be adjusted in the direction of full displacement without any initial decrease in displacement.
In the constant horsepower control mode the mechanical advantage is caused to increase even though this correction is in the wrong direction to correct for the speed decrease. So long as the pressure remains at or above the predetermined set pressure level (as set by an adjustable pressure setting control) the mechanical advantage of the motor continues to increase for the purpose of delivering increased torque to the motor output shaft. Stated another way, the constant horsepower control mode overrides the speed control mode to allow the motor speed to drop to a level at which sufficient torque can be developed to overcome the increased motor load providing constant output horsepower and then once sufficient motor torque is being delivered, the speed will stabilize and no further increases in displacement will be made. If the load subsequently begins to decrease, the motor shaft speed is caused to increase back toward the set speed with the motor providing constant horsepower output whereby the motor operates at the largest speed below the set speed at which it can deliver the required torque. Presuming that the load returns to its original level or lower, the control reverts to the speed control mode once the shaft speed equals the set speed.
If instead of the load dropping, it continues to increase to a maximum and the motor is still unable to satisfy demand at the set pressure, then the motor stalls.
In a preferred form of control system which is associated with a hydraulic motor pursuant to this invention, adjustable speed and pressure setting controls are provided for use by an operator. There are also provided with the control, and not accessible by an operator, a preset maximum pressure limit and a preset maximum speed limit to establish internal command limits which prevail over any other commands which would exceed said limits so that actual pressure and speed are always limited to the mechanical ratings of the motor or to a lower rating of the particular usage of the motor
Briefly, the control system of the present invention applied to one embodiment to a variable displacement motor of the type described above comprises an electronic control circuit which controls a variable control valve which in turn controls the hydraulically actuated control pistons for controlling the mechanical advantage of the hydraulic motor. The electronic control circuit receives input signals from transducers, or sensors, associated with the motor. One sensor is a pressure transducer which senses the fluid pressure in the system. Another is the tachometer which senses motor shaft speed. These sensors and their associated conditioning circuits develop signals representative of pressure and speed respectively, and the electronic control circuit acts upon these signals in controlling the variable control valve.
The speed and pressure settings which in the disclosed embodiment are controllable by an operator comprise respective potentiometers which are adjustable to establish respective signals representative of desired speed and pressure, pressure being indicative of load. For a person operating the control, the speed control setting is invariably labeled in terms of speed, and the pressure control setting is usually labeled in terms of pressure. Because pressure, at a given flow and displacement, represents torque and horsepower, a control labeled in a parameter other than pressure could be displayed to the operator for the load setting control.
The maximum pressure and maximum speed limit controls are also potentiometers in the preferred disclosed embodiment and they establish the maximum permissible pressure and speed respectively. The electronic control circuit is effective to prevent the motor from exceeding a pressure or a speed above the maximum allowable levels as set on these latter potentiometers even though other signals may be commanding otherwise.
The electronic control circuit is organized and arranged in a speed control loop and a pressure control loop, both of these being in the nature of closed feedback loops which receive the sensed speed and pressure signals and act upon them in certain relationships with the speed and pressure settings of the potentiometers to control the variable control valve and hence the mechanical advantage of the motor. The two loops however, an alluded to above, have an interaction whereby the control transfers from one loop to another under certain operative conditions so that at certain times the speed control loop provides control to the exclusion of the pressure control loop while at other times the pressure control loop provides control to the exclusion of the speed control loop. There is also a further minor loop around the variable control valve to insure the best correspondence of the valve to the commanded position.
Organization of the control electronics is such that its basic design may be adapted for different sized motors by merely changing the values of certain individual components. The electronic control is also arranged such that it may be packaged for use as original equipment with an original equipment motor, or on existing motors as replacement of the existing controls.
The preferred embodiment also comprises the advantage of bi-directional motor operation and control. In the preferred embodiment this capability is accomplished by means of a shuttle valve which is connected across the motor ports and which is associated with the pressure transducer so that the action of the control is effective regardless of flow direction. An ancillary aspect of this arrangement of the hydraulic circuit and control is that the control becomes inherently effective to retard the motor should the motor overrun.
Many applications in which hydraulic motors are used do not impose constant load demands. For example, consider an anchor winch on a ship. The initial operation to raise the anchor requires maximum torque in order to overcome the inertial force of the anchor and paid-out line and any resistive forces acting on them. As the anchor is towed in, the torque demand will generally decrease. To take advantage of the reduced torque demand, speed can be increased along a constant horsepower curve until such time as the maximum allowable winding speed is reached. With prior types of manual controls, it is essentially impossible to satisfactorily manually adjust the controls so as to reel in the anchor in minimum time. With the present invention the reel-in time can be minimized while insuring a maximum reel-in speed within a desired speed limit.